1. Field of the Invention
The present invention relates to a sealed hydraulic intensifier and, more specifically, to a sealed hydraulic intensifier which is effectively applicable to a mold clamping mechanism for an injection molding machine or the like.
2. Description of the Related Art
In a mold clamping mechanism included in an injection molding machine, a movable plate is moved to clamp a mold between the movable plate and a fixed plate and to open the mold. The mold clamping mechanism must be able to clamp the mold by a high mold clamping force to keep the mold closed against a high injection pressure applied to a molten polymer in the mold. Conventional mold clamping mechanisms having such a function include mold clamping mechanisms of a boost ram system having a boost ram inserted in the clamping ram of a clamping cylinder actuator, mold clamping mechanisms of a side cylinder actuator system having a plurality of side cylinder actuators in addition to a clamping cylinder actuator, and mold clamping mechanisms of a pressure intensifying cylinder actuator system provided with a pressure intensifying cylinder actuator connected in series to a clamping cylinder actuator. A straight hydraulic mold clamping mechanism capable of quickly closing and opening a mold and of clamping the mold by a high pressure has widely been used.
A conventional straight hydraulic mold clamping mechanism needs a complicated hydraulic power system using clamping cylinder with high capacity to close and open the mold quickly and to exert a sufficiently high clamping pressure to the mold, and needs a fluid tank for containing working fluid to be supplied to a clamping cylinder actuator. Particularly, the straight hydraulic mold clamping mechanism of a boost ram system and that of a side cylinder system need a high-pressure pump to apply a high clamping force to a mold. The straight hydraulic mold clamping mechanism of a pressure intensifying cylinder actuator system has a relatively great length.
A prior art mold clamping mechanism having a combination of a ball screw and cylinder is disclosed in JP-B No. 89-17851.
This prior art mold clamping mechanism is provided with a first double-acting cylinder having a first and second pressure chamber and a second double-acting cylinder having a booster chamber. The first and second pressure chamber have volume which vary by the same volume according to the movement of a double-acting ram. In the clamping mechanism, a valve is operated so as to connect the pump to the first and the second pressure chamber, and the pump is driven to operate the first double-acting cylinder actuator to open and close the mold. The valve is operated so as to disconnect the pump from the first and the second pressure chamber, and the screw mechanism is driven to operate the second double-acting cylinder actuator for a mold clamping operation.
When the mold clamping mechanism operates to open (or close) the mold, the working fluid filling up the first and the second pressure chamber and the liquid passage is driven for flowing by the pump to advance (or retract) the double-acting ram. Therefore, the mold clamping mechanism does not need any external tank for storing the working fluid.
A conventional mold clamping mechanism is provided with a screw mechanism driven by an ac servomotor to move a movable plate in opposite directions. The servomotor and the screw mechanism for transmitting a mold clamping force of this mold clamping mechanism need inevitably to be large to apply a sufficiently high clamping force to the mold.
Another prior art mold clamping mechanism for an injection molding machine, proposed in JP-A No. 94-246806 comprises a improved screw mechanism having a nut provided with an internal screw thread and a screw shaft linked to the nut; a servomotor for driving the nut of the screw mechanism.
This prior art mold clamping mechanism is provided with a clamping force intensifying mechanism having the screw shaft engaging with the nut driven by the servomotor. When the screw shaft compresses the liquid sealed in the sealed bag, enhanced pressure of the liquid sealed in the sealed bag is produced as a high clamping force.
However, the intensifying mechanism has such disadvantages that hydraulic fluid may leak from the sealed bag and the high clamping force can not be securely maintained.
Accordingly, it is an object of the present invention to provide a sealed hydraulic intensifier which has a simple and compact mechanism and a high degree of flexibility of design.
It is an another object of the present invention to provide a sealed hydraulic intensifier which achieves an appropriate intensifying action to maintain a high clamping force to a predetermined value and is securely supplied with hydraulic fluid, and free from hydraulic fluid leakage.